Methods for electroplating metallic tin coatings onto electrically conductive coatings have been well known in the art for many years. Immersion plating of metallic tin onto metallic substrates by ion displacement of tin ions for the parent metal atoms is also a common industrial process. The book by F. A. Lowenheim (Modern Electroplating, The Electrochemical Society, Inc., Pennington, N.J.) provides an in-depth survey of both techniques and an extensive list of pertinent references and patents.
Although the mechanisms for true electroless plating of tin onto nonconductive substrates have been reported (e.g. (1) U.S. Pat. No. 3,063,850 to Mikulski; (2) M. E. Warwick and B. J. Shirley, Trans. Inst. Met. Finish, 58,9 (1980), (3) A. Molenaar and J. J. C. Coumans, Surf. Tech., 16,265 (1982), (4), A. Molenaar, Extended Abstracts, Vol. 84-2, The Electrochemical Society, Pennington, N.J., 634 (1984), and (5) H. M. Van Noort et al., J. Electrochem. Soc. 133,263 (1986)), the quality and reproducibility of the films are often poor and appear to arise from the self-catalyzed reaction EQU 2 Sn(II).sub.(aq) =Sn(IV).sub.(aq) +Sn(0).sub.s
since other reducing agents are not required (but may participate to some extent; e.g. V(II), Cr(II)). The use of Sn(II), notably SnCl.sub.2, as a sensitizing/catalyzing agent in electroless deposition of other electropositive metals such as palladium is also standard practice (e.g. U.S. Pat. No. 4,554,182 to Bupp et al., and prior art cited therein), but is not directly related to Sn(0) thin film formation.
Electroless deposition of nonmetallic semiconducting or insulating compounds, especially metal sulfides and selenides, has been reported in the literature (e.g. (1) P. Pramanik and S. Biswas, J. Electrochem. Soc., 133,350 (1986), (2) S. Biswas, et al., J. Electrochem. Soc., 133,48 (1986), (3) R. A. Boudreau and R. D. Rauh, J. Electrochem. Soc., 130, 513 (1983), (4) R. N. Bhattacharya and P. Pramanik, J. Electrochem. Soc., 129,332 (1982), and (5) K. L. Chopra et al., Physics of Thin Films-Vol. 12, Academic Press (1982), and references contained therein.) In all cases, the nonmetal (S, Se)=X reagent is a S or Se-containing compound in which S,Se exists in the anionic state and which slowly decomposes/disproportionates into S.sup..dbd. or Se.sup..dbd. without involvment of reducing agents or catalysts. The metal chalcogenide film is formed by precipitation of these generated chalcogenide anions and metal cations released by the slow disproportionation of metal complexes (chelates). This precipitation is energetically favored on solid surfaces that provide a bounty of nucleation sites. The continuous and very slow precipitation of these ions, when the M.sub.x X.sub.y solubility product is exceeded, leads to growth of macroscopic films with thickness of order of 0.1-1 .mu.m.). The technique (e.g. Reference 5 above) appears applicable to nearly all metal chalcogenides in which the metal has a chelate that can slowly release free metal cations.
Major disadvantages of prior art electroless compound plating include (1) The use of relatively expensive and normally toxic sulfur/selenium compounds where X.sup..dbd. already exists as a branch of the molecule, (2) Slow deposition rates and small terminal film thicknesses due to the slow formation of X.sup..dbd. ions, (3) Tendency toward amorphous film formation due to the low growth rates, and (4) The requirement of careful control of pH and metal ion complexing agents.
Weakly adsorbed films of collodial particles of metal compounds can be produced by the method described in S. M. Kulifay, Am. Chem. Soc., 23,4916 (1961) in which metal cations (e.g. Te.sup.+4, HSeO.sub.3.sup.-, AsO.sub.3.sup.-, SbO.sup.+) of the "nonmetals" tellurium, selenium, arsenic, or antimony, and strong reducing agents such as hydrazine (N.sub.2 H.sub.4), methanol, etc. are dissolved in aqueous solutions and heated. The separate reducing agent reduces both the metal and nonmetal ions to solid particles which subsequently react to form M.sub.x X.sub.y particles which can stick/adsorb (not nucleate onto) to solid surfaces.
Small numbers of nonmetal anions (e.g. Te.sup..dbd.) may be directly generated by the strongly favored electron transfer from the reducing agent to the electropositive nonmetal ion (e.g. HTeO.sub.2.sup.+). In this case, the precipitation/nucleation reaction can again occur directly on the solid surfaces EQU (e.g. Cd.sup.++.sub.(aq) +Te.sup..dbd..sub.(aq) =CdTe(s)).
Experiment has shown that most of the reactants form powder suspended in solution.
This method is not satisfactory for producing adherent, strong deposits. Major disadvantages include (1) Requirement of a separate, strong, and usually toxic/flammable/explosive chemical reducing agent (e.g. N.sub.2 H.sub.4, CH.sub.3 OH), and (2) Film formation only by adhesion/occlusion of colloidal particles formed in the solution and not by true ion/ion condensation and nucleation and, thus, very weak and nonuniform films.
Chemical vapor deposition, another standard, common-knowledge technique of producing compound/alloy films, involves the reaction of multiple gases or vapors containing the film elements on the surface of a heated substrate. These compressed, ultrapure gases are fed into an evacuated or inert reaction chamber via expensive, corrosion-resistant, and fail-safe plumbing connected, through appropriate controls, to purchased cylinders. Normally, these gases are extremely toxic and expensive. For example, arsine (AsH.sub.3) and trimethylgallium ((CH.sub.3)Ga) can be simultaneously fed into a chamber where they react on a hot (700.degree. C.) substrate according to EQU AsH.sub.3 +(CH.sub.3).sub.3 Ga=GaAs+3CH.sub.4.
The books by Chopra and Das (Thin Film Solar Cells, Plenum Press, New York, 240-256 (1983)) and Vossen and Derr (Thin Film Processes, Academic Press, New York (1978)), and references therein, include surveys of the state of the art in chemical vapor deposition as well as other thin film deposition techniques. Major disadvantages in the prior art of chemical vapor deposition include (1) The very high (several hundred .degree.C.) temperatures normally required to activate the chemical reactions, (2) The use of externally stored and fed toxic/corrosive gases, and associated safety problems, (3) The required extreme purity of the gases, (4) Need for evacuation or inert gas flooding of the reaction chamber, and (5) The great expense of the process due to (1) through (4).
By spraying a fine mist of solutions of strong reducing agents and metal/nonmetal salts onto hot (T 300.degree.-700.degree. C.) substrates, the solvent is nearly instantaneously vaporized and the chemical reduction of the salts to the elements and their subsequent reaction to form the compound are strongly enhanced due to the temperature. The resultant films are much more adherent, uniform, and crystalline than in the wet bath procedure of Kulifay. However, in addition to the disadvantages listed under Kulifay, this apparatus includes a pressurized solution vessel, chemical tubing, inert gas propellant, heater etc., all relatively expensive. The book by Chopra et al., (Physics of Thin Films-Vol. 12, Academic Press (1982)) reviews this "spray pyrolysis" procedure. Hill and Chamberelin (U.S. Pat. No. 3,148,084) were pioneers in this area.
Schneble (U.S. Pat. No. 4,027,055) discloses a method for depositing smooth and easily solderable tin metal coatings onto metallized surfaces from aqueous baths containing a soluble stannous salt, a sulfur component which comprises a mixture of alkali metal polysulfides and at least one other sulfur-containing compound, a mineral acid, and a wetting agent. The technique is basically an ion displacement/cementation (immersion) process where the metal substrate provides the electrons required for Sn.sup.++ reduction. It does not work on nonconductive substrates. Schneble found that polysulfide ions (S.sub.x.sup..dbd.) generated via disproportionation of the added sulfur compounds (e.g. thiourea) were necessary to produce high quality tin films. Quite likely, S.sub.x.sup..dbd. x=simply complexed Sn.sup.++ and, hence, slowed down the ion displacement rate, served as a surface blocking agent (brightener), and produced bright, smooth deposits, as is commonly found with complexing/brightening agents during electro- or electroless deposition of metals.
Schneble did observe pale green and coffee colors upon heating his solutions, probably due to formation of SnS.sub.2.sup..dbd..sub.(aq) and SnS.sub.(s), respectively, within the solution. He speculates that the coffee color is believed to be due to the formation of a precipitate, stannous sulfide. "The precipitate will do-deposit on any parts being plated during the transitional period, causing grey-black deposits and occasionally a rust-colored dusty deposit. These deposits can easily be removed by a light brushing of the part with water." Evidently these so-called "deposits" are simply adhesions/occlusions of the precipitate easily predicted to be formed within the bath and not a true adherent deposit. Furthermore, Schneble makes it more than clear that this SnS formation is not desirable to his process and discusses ways to avoid or circumvent it. He does not mention Sn.sub.x S formation and does not discuss the controlled formation of other Sn.sub.x S phases and associated multiple layer devices. Furthermore, he, in all cases, utilized sulfur compounds in aqueous solutions and nowhere mentions elemental sulfur molecularly dissolved in predominantly organic acid solutions.
Howes (U.S. Pat. No. 2,757,104) discusses a process to form electrical resistors from alloy films of predetermined and predictable electrical characteristics. The process involves deposition of a "primary" metallic film onto a substrate by any suitable method (electroless deposition, spraying, evaporation or sputtering). The precision alloy is formed by submerging the primary film in a bath containing more electropositive ions of another metal. These ions exchange for atoms of the primary film by ion displacement and electron exchange. This exchange will shift the open circuit voltage of the films positively until the Nernst potentials of the primary film and ion elements become equal and the process ceases. This should correspond to a well defined composition and, hence, resistivity. Subsequent annealing can stabilize and oxidize the films, producing higher resistivity metal oxide resistors.
Howes' process is an ion exchange process dependent upon the pre-existence of a primary metal film. The alloy can not be deposited on a nonconductive substrate in a single step due to this requirement. The ion exchange reaction requires ions more electropositive than the primary film. Howes specifies silver as the primary film and this restricts the ions to those of nobler but often expensive elements such as palladium. Howes makes no mention of tin metal or tin sulfide formation. Since sulfur has no simple ionic cations capable of undergoing this ion exchange, metal sulfides can not be formed.
Byatt (U.S. Pat. No. 4,375,125) discusses the formation of a semi-insulating, passivating metal oxide, metal chalcogenide, or silicon layer on pre-existent p-n junctions (notably silicon) by "generic" chemical conversion treatments (nonspecific). This patent teaches one of a multitude of applications of metal chalcogenides and one of a multitude of ways to improve p-n junction electrical characteristics, but does not deal specifically with improvement in the art of metal chalcogenide, notably Sn.sub.x S, film deposition or the creation of p-n junction themselves from metal chalcogenides such as Sn.sub.x S.